It is known that, in a device that controls a motor that drives an industrial machine, torque pulsation is caused depending on a rotational position (a rotation angle) of a motor according to the characteristics thereof, and the torque pulsation is called “torque ripple”.
For example, in a permanent-magnetic synchronous motor, a cogging torque is caused due to distortion of magnetic-flux changes inside the motor, and the number of times of pulsations (also referred to as “the number of peaks”) determined by the configuration of the motor (the number of poles or slots) for one rotation thereof (also referred to as “mechanical angle”) are caused. Such a torque ripple sometimes causes a negative influence on operations of the machine, and therefore there has been proposed a method for suppressing the torque ripple by a control device.
As a control device that suppresses a torque ripple, there has been known a device that offsets a torque ripple by a correction torque command given at an equal angular period according to a rotational position in consideration of the torque ripple occurring periodically depending on the rotational position. An amplitude and a phase of the caused torque ripple are different for each motor depending on variations and the like generated during manufacturing of motors, and therefore it is necessary to also set an amplitude and a phase of the correction torque command for each motor.
As described above, as a control device that suppresses a torque ripple by determining an amplitude and a phase of a correction torque command for each motor, for example, the following techniques have been proposed. That is, there has been disclosed a technique including a step of changing a phase of a correction torque command indicating a sine wave over the whole range (0 to 360 degrees) at a predetermined pitch width, and a subsequent step of changing the amplitude of the correction torque command at a predetermined pitch width, by which a magnitude of the torque ripple is analyzed by an FFT computation unit every time the phase and the amplitude of the correction torque ripple are changed, thus determining the amplitude and the phase of the correction torque command that minimize the torque ripple (see, for example, Patent Literature 1).
Further, there has been disclosed a technique in which a sampling unit that samples, under a set condition, a post-correction torque command that is a signal to which the correction torque command has been added, an FFT computation unit that calculates a Fourier coefficient by an FFT computation, and a correction-value computation unit that performs computations on a correction value based on the Fourier coefficient, are provided, and a step of sampling performed by the sampling unit and a step of determining the Fourier coefficient of the sampled post-correction torque command and updating the correction torque command are performed for a set number of repetitions, thereby calculating a torque-ripple correction value (see, for example, Patent Literature 2).